Semiconductor devices are tested, often multiple times, during their manufacture. A piece of automatic test equipment, referred to as a “tester,” is used to generate test signals that stimulate a device under test (DUT) and to measure the response. The tester determines whether a DUT is operating properly by comparing the response evoked by a carefully controlled test pattern with an expected response.
To fully test devices, the tester should generate and measure signals such as may be found in the operating environment of those devices. Increasing complexity of semiconductor chips has required that automatic test equipment also generate and measure more complex signals. Most semiconductor devices generate or respond to high speed digital signals. Many devices, such as disk drive controllers and processors for video signals, also generate or respond to analog signals. Entire systems, containing both analog and digital electronics, are now widely implemented on single semiconductor devices.
Automatic test equipment must now generate both digital and analog signals. Accordingly, test equipment is typically made to contain multiple instruments. Each instrument performs a specific function, such as generating high speed digital signals or producing an analog waveform that has a programmed characteristic. Multiple instruments are installed in a tester to provide the combination of analog and digital signals needed to test a particular device. Creating instruments that provide separate test functions provides a flexible way to create a test system that can generate and measure a set of test signals required for testing virtually any semiconductor device.
However, assembling a test system from separate instruments creates an additional challenge for test system designers because the actions of the various instruments must be coordinated. For a test system to properly evaluate test results on a semiconductor device, it is often necessary for the tester to determine both that a specific signal was detected and that the signal occurred at a specific time in relation to a certain stimulus. Coordinated operation of the instruments is necessary for signals to be generated and measured with specific time relationships.
One way to coordinate instruments is to provide centralized circuitry that provides a reference clock and commands to all instruments. A circuit in a tester that provides a series of commands to control the generation and measurement of test signals is called a “pattern generator.”
There is often a practical limit to the frequency of a reference clock that can be reliably fanned out to many instruments in a test system, which can be undesirable. Events that are timed relative to edges of a clock may be specified with a resolution limited by the period of the clock. Lower frequency clocks have longer periods and therefore provide less timing resolution.
Where greater timing resolution is desired, it is known to use an “interpolator.” An interpolator is a circuit that can track an interval that is a fraction of a period of a clock. However, interpolators must be accurate and stable. Designing and building interpolators in a test system therefore presents complexities not present when times are measured relative to a digital clock.
A variation on the approach of using a centrally created clocking architecture is employed in the Catalyst™ mixed-signal semiconductor test system, manufactured by Teradyne, Inc., of Boston, Mass. The architecture is shown generally in FIG. 1 and includes a reference clock generator 8 that generates a clock that is distributed, or fanned out, to a plurality of digital and analog channel cards 10 and 12, respectively. Each analog or digital card may be considered a separate instrument, though it should be appreciated that an instrument is a logical concept and that an instrument may be implemented on multiple circuit cards or, alternatively, may be implemented on a single circuit card along with other circuitry.
Signals generated by a centralized pattern generator 14 are fanned out with the reference clock to the channel cards. Pattern generator 14 issues commands that are to be performed by each instrument. A command may be generated for each instrument for each cycle of the reference clock.
Clock signals for the digital cards are fed to timing circuitry 16, which drives waveform formatting circuitry 18 to produce digital signals for application to the device-under-test (DUT, not shown). The analog cards 12, on the other hand, receive the remotely generated digital reference clock signal and synthesize an analog clock through analog clock module (ACM) 19. The local analog clock AO drives functional circuitry on one or more analog instruments.
One form of the analog clock is described in U.S. Pat. No. 6,188,253, entitled Analog Clock Module, assigned to the assignee of the present invention, and expressly incorporated herein by reference in its entirety. Each analog instrument may have its own clock and therefore operate at its own frequency, which could be higher than the frequency of the reference clock.
In a variation of the design shown in FIG. 1, each instrument includes a pattern generator. The pattern generators operate synchronously, based on the reference clock signal. Each pattern generator outputs commands or “events” for its specific instrument at the required time.
A further variation is for each instrument to include a local clock generator to drive its own pattern generator. The local clock generator may produce clocks of different frequencies. However, it is necessary that the pattern generators start in a coordinated fashion.
Published patent application WO/03042710 entitled “CLOCK ARCHITECTURE FOR A FREQUENCY BASED TESTER” (which is hereby incorporated by reference in its entirety) describes a system for coordinating the operation of pattern generators operating at different frequencies. The approach in that published application employs a synchronization signal, called DSYNC, in connection with a reference clock to “align” all of the local clocks at a specific time.
A need exists in the art for a test system in which operation of multiple instruments is readily synchronized.